Optical technology allows high speed wireless communication. Optical signals can be carried at very high bandwith and low noise. Such signals can also travel for much longer distances, e.g. on the order of kilometers. Many optical receivers are used to receive signals that have been sent over tens of kilometers. Since the signal is received in optical form, there are also fewer problems with ground loops, cross talk and noise.
Photoreceptors for optical signals are often formed in non-standard substrates such as InP or GaAs to obtain the high sensitivity that is necessary for receiving optical signals that have traveled over the long distances. These substrates are often incompatible with the CMOS substrates used for many other IC processes, e.g. microprocessors.
Optical transmitters similarly have been formed in non-standard substrate types and materials. In addition, optical transmitters often require substantial power to be generated on the optical chip, e.g. to generate the light for transmission.
Some optical transmitter configurations form a semiconductor laser on the chip. However, these systems are also relatively difficult to reliably manufacture.
The high bandwidth capability of optical signals allows the medium to carry much information.
Modern processors require high data input and output. Typical connections to a processor are made in parallel to provide the required throughput. This requires large numbers of connection pins. For example, a 128-bit processor may use 128 input lines for the data input. As processors become more powerful, it becomes increasingly difficult to provide enough pins to allow the desired connections.
The high bandwidth of optical technology could allow serial communication at much higher speed over fewer lines. This could reduce the pin count. However, the non-standard semiconductor technology has made this an unattractive option.
Modern chip fabrication is often done in CMOS. If a different kind of fabrication technology is used for the optical sensor or transmitter, that optical sensor or transmitter is preferably formed on a totally separate substrate from the CMOS sensor.
The power consumption of the optical transmitters makes it even more unattractive, since processors already have extreme power consumption and cooling requirements. Adding additional power consumption is undesirable.